Power that is delivered from storage devices, such as batteries, is delivered in the form of a direct current (DC) while power that comes from a power plant uses alternating current (AC). The positive and negative connectors on a DC battery send voltage in one direction, with negatively charged electrons being attracted toward a positively charged connector at the other end of the wire and vice versa, and thus remain positive and negative at all times. With AC transmissions, power flows in two directions, meaning that the terminals change polarity depending on the direction of the power flow. The frequency with which AC current changes direction is measured in Hertz (Hz); in the United States, current is delivered using a frequency of 60Hz.
A big advantage that AC electricity has over DC electricity is that it can be easily transformed from a high voltage level to a low voltage level using a device known as a transformer. The cables used to transmit electricity over long distances resist that flow, so high voltages have to be used to push the electricity along them. These voltages would be dangerously high if they came into homes or business environments, so transformers are used to lower the voltage of the electricity before it is delivered to the end user. In the United States, electricity is delivered to end users at 120 volts.
Cables are less resistant to DC current so that lower voltages can be used to move power along them than is required with AC transmission over the same distance. However, for the distances that AC power currently travels from power plants to end users, cable resistance to DC current would be such that much higher voltages than end users require would have to be used, and the process for lowering DC voltage is much more complex and therefore expensive than it is for AC electricity.
When electricity flows down a cable, it generates an electro-magnetic field. When the current changes, as it does with AC current, a counter electro-magnetic field is produced that acts as a resistance to the power being transmitted. These means that AC transmission of electricity loses power due to both resistance and to reactance. Because DC power transmission never changes direction, it is not susceptible to power loss as a result of reactance.